To improve illumination result and increase input electricity of discharge lamps, the driving method of using a push-pull inverter to ignite a discharge lamp by providing electricity at a phase difference of 180 degrees on two input electrodes at two ends of the discharge lamp has become an important technique. U.S. Pat. No. 6,087,757 discloses such a technique. It has a control unit to provide a driving frequency which alters input electricity to a transformer by 180 degrees of phase difference through a phase inversion circuit. U.S. Pat. No. 6,724,126 also discloses a technique which has a control unit to output frequencies of the same phase and two transformers of opposite polarity to generate input electricity at a phase difference of 180 degrees.
While the aforesaid patents have included discussion of the push-pull inverter, the two ends of the discharge lamp receive input electricity from the inverter. The discharge lamp cannot feed back the actual driving electricity condition through a coupling circuit to the control unit. In fact, after the discharge lamp has been used for a period of time, the discharge gas ages or input electrodes oxidize. As a result, the luminance of the discharge lamp becomes not uniform. If there is no feedback electricity sending timely to the control unit to adjust the driving frequency, aging of the discharge lamp will accelerate.
U.S. Pat. No. 6,087,757 provides a solution which connects the output side of the transformer to a lamp current detection device to get a feedback electricity detection value. It adopts a contact circuit which is not adaptable to the push-pull inverter.